The present invention relates to a method of forming fiber gratings used as components for various types of optical sensors and for optical transmission, and a fiber grating formed by the method.
As already known, for example, in the field of optical transmission, an optical fiber in which bare optical fibers are jacketed at the outer circumference is used, and the bare optical fibers are formed by covering the core thereof, which is an optical transmission path, with cladding. A fiber grating is such that the core of an optical fiber is made of, for example, germanium (Ge) doped quartz (SiO2) glass, the refractive index is increased by irradiating intensive ultraviolet rays to the core, whereby cyclic refractive index changes arise in the beam axis of the optical fiber, and a refractive grating is formed.
The fiber grating functions as a reflection filter which selectively reflects only light if a specified wavelength, which is predetermined. For example, the fiber gratings are used as a division element, variable wavelength filter, a wavelength dispersion compensating element, sensor element etc., which are capable of dividing wavelengths by reflecting light of the specified wavelength from light which is multiplexed for wavelength division transmission.
As a method of forming fiber gratings, for example, a phase mask method, and holographic method are known. The phase mask method is such that ultraviolet rays are irradiated on optical fibers from above a phase mask to produce or form fiber gratings. For example, this method is described in a Journal Appl. Phys. Lett., 62, 1035. 1933, etc.
In the case of forming fiber gratings using the phase mask method, pressure hydrogen treatment is applied to an optical fiber in order to improve optical induction characteristics. As shown in FIG. 2, a sheath 2 of the optical fiber is partially removed to expose a predetermined length of the bare fiber 1. Next, as shown in the same drawing, for example, ultraviolet coherent light (ultraviolet light) emitted from a laser, etc., for ultraviolet ray irradiation, which is provided at a side of the optical fiber 3, is reflected by a mirror after being passed through a slit 5, and is condensed by a cylindrical lens 16. Thereafter, the light is caused to pass through a phase mask 9 and is irradiated onto a grating forming area 6 (fiber grating forming area) of the bare optical fiber 1.
In addition, a position where the laser is arranged is not specially limited. However, as shown in, for example, FIG. 4, there is a case where an excimer laser 11 is provided in a direction orthogonal to the beam axis of the exposed bare optical fiber 1, and after an ultraviolet coherent ray emitted from the excimer laser 11 is condensed by the lens 10 without providing a mirror 8 secured in FIG. 2, the condensed ray is irradiated onto the grating forming area 6 of the bare optical fiber 1.
In either case, as ultraviolet rays are irradiated on the bare optical fiber 1 through a phase mask 9, an intensively irradiated portion and a softly irradiated portion of ultraviolet rays are formed on the bare optical fiber 1, wherein at the intensively irradiated portion of ultraviolet rays, a bonding of Gexe2x80x94Si of the core is frequently cut off, a ratio of increase in the refractive index is increased, and at the softly irradiated portion of ultraviolet rays, a bonding of Gexe2x80x94Si of the core is hardly cut off, or no change in the refractive index occurs. Thus, by alternately forming a high portion of the refractive index and a low portion thereof, a multilayered structure is formed, in which the refractive index may differ in the lengthwise direction of optical fibers (that is, the refractive index of the core cyclically changes in the beam axis of the optical fibers).
During irradiation of ultraviolet rays, as shown in, for example, FIG. 4, since the optical fiber 3 is held by an optical fiber holding portion 13, the bare optical fiber 1 is held with a stress applied thereto, whereby the frequency of changes in the refractive index of the fiber grating does not slip.
Further, since a device shown in FIG. 4 repeats operations of an operation stage 12 by a stepping motor 14 after a grating is formed at a part of a bare optical fiber 1 by an excimer laser 11 and actions of forming another grating at different portions of the bare optical fiber 1, a fiber grating can be formed at a wide area of the bare optical fiber 1.
After the irradiation of ultraviolet rays is finished, there are many cases where heat is partially provided to the grating formed area 6 to remove defects in glass which are thermally unstable. In addition, the heat treatment is carried out by heating the bare optical fiber 1 at several tens of minutes at approx. 200xc2x0 C. Further, in order to secure long-term reliability in the strength of optical fibers, the grating formed areas 6 are accommodated in a production sleeve such as, for example, a stainless steel pipe, or the sheath 2 is reproduced on the surface of the grating formed areas 6. Also, in a case where a grating formed area 6 is formed at the distal end of an optical fiber 3, the grating formed area 6 may be housed in, for example, a connector.
In a case where a fiber grating is formed by using a holographic method, interference light which is produced by interfering two coherent ultraviolet rays is irradiated on an optical fiber instead of forming an intensively irradiated portion and a softly irradiated portion of ultraviolet rays with a phase mask 9 provided, whereby an intensively irradiated portion and a softly irradiated portion of ultraviolet rays are formed, thereby forming a fiber grating.
Also, in cases of using a phase mask method as described above and cases of using the holographic method, basically, a series of processes consisting of removal of the sheath of the optical fiber 3, formation of gratings by irradiation of ultraviolet rays, heat treatment, and protection of grating formed areas 6, are carried out in order to form fiber gratings.
In a process of forming fiber gratings by performing the series of processes, it is unavoidable that an exposed bare optical fiber is brought into contact with other components, where dust is adhered to the bare optical fiber, or the glass surface of the bare optical fiber 1 is damaged.
Further, for example, moisture in the air may be provided to the bare optical fiber 1. As described above, when irradiating ultraviolet rays, since the optical fiber 3 is held by a predetermined tension applied to the bare optical fiber 1, stress corrosion may be produced by the humidity in the air when irradiating ultraviolet rays. Still further, in a fiber grating formed component, there is an optical component which holds both ends of the fiber grating formed area by metal with tension applied to the bare optical fiber 1 after irradiation of ultraviolet rays is finished, in order not to cause the frequency of changes in the refractive index to slip due to elongation and contraction of the fiber grating due to temperature changes. In such an optical component, stress corrosion may arise due to moisture after the irradiation of ultraviolet rays.
Therefore, such a problem occurs, by which the strength of portions is decreased, where the surface of the bare optical fiber 1 is damaged or stress corrosion arose, and the long-term reliability of optical fiber type optical components in which fiber gratings are formed will be lowered.
That is, in order to secure long-term reliability of components, a screening test is carried out, by which inferior portions incapable of standing against a load when the load necessary for a reliability design is applied are removed. As fiber gratings are formed by using a conventional forming method of fiber gratings, there are many cases where the fiber grating areas 6 may be broken by applying to the areas 6 only a load equivalent to 1.0 GPa or the like used for a comparatively low level screening test. Resultantly, the yield of products of optical fiber type optical components in which fiber gratings are formed was remarkably low.
Also, quartz which forms a bare optical fiber 1 may change in length due to temperature change, and another problem arises, in which the reflection wavelength band of fiber gratings shifts in line therewith. To avoid the problem, recently, tension is always applied to the bare optical fiber 1, wherein, when the bare optical fiber 1 is elongated in line with a temperature rise, the tension is decreased, while the tension is increased when the length of the bare optical fiber 1 is contracted in line with a temperature drop. Accordingly, such a method has been employed, by which the reflection wavelength band of fiber gratings can be kept constant by adjustment of the tension.
In order to employ this method for optical fiber type optical components, it is necessary to apply a load equivalent to 1.5 through 2.0 Gpa or the like to the fiber grating formed area 6 in the abovementioned screening test. However, if so, in optical fiber type optical components in which fiber gratings are formed by using the conventional fiber grating forming method, it can be said that nothing exists which can pass this level of screening test.
The present invention was developed in order to solve the conventional problems and shortcomings. It is therefore an object of the invention to provide a method of forming fiber gratings, by which strong fiber gratings having high reliability can be formed, and fiber grating formed optical components can be produced at a high yield, and to provide fiber gratings formed by this method.
In order to achieve the above object, the invention is constructed so that the following construction is used as means for solving the problems and shortcomings. That is, the first aspect of a method of forming fiber gratings, according to the invention, comprises the steps of covering the surface side of a bare optical fiber with a resin film having a thinness to obtain an ultraviolet ray permeability which does not hinder formation of the gratings; and thereafter forming gratings by which the refractive index of an optical waveguide of the bare optical fiber can cyclically change in the beam axis by irradiating ultraviolet rays from the surface side of the corresponding resin film.
Further, the second aspect of a method of forming fiber gratings, according to the invention, comprises the steps of partially removing the sheath of an optical fiber formed by providing the outer circumference of a bare optical fiber with a sheath; covering the surface side of the bare optical fiber at the areas, where the sheath is removed, with a resin film having a thinness by which an ultraviolet ray permeability not hindering formation of the gratings can be obtained; and thereafter forming gratings by which the refractive index of an optical waveguide of the bare optical fiber can cyclically change in the beam axis by irradiating ultraviolet rays from the surface side of the corresponding resin film.
In addition, the third aspect of a method of forming fiber gratings, according to the invention, comprises the steps of covering the surface side of a bare optical fiber with a resin film having a thinness to obtain an ultraviolet ray permeability which does not hinder formation of the gratings; thereafter providing a sheath for the outer layer at the outer circumferential side of the optical fiber on which the resin film is coated, and thereafter forming gratings by which the refractive index of the optical wavelength of a bare optical fiber can cyclically change in the beam axis by partially removing the sheath of the outer circumference of an optical fiber ribbon in a case where a bare optical fiber is provided with gratings by irradiating ultraviolet light from the surface side of the exposed resin film.
The invention is also featured in that the resin film is made of organic materials which are not dissolved by the ultraviolet ray heat irradiation heat.
In addition, the invention is also featured in that the resin film is composed of an organic material having heat resistance, by which the resin film is not melted by ultraviolet ray irradiation.
Further, a fiber grating, according to the invention, is featured in a grating, in which a sheath of an optical fiber formed by providing the sheath on the outer circumference of a bare optical fiber is partially removed, the surface side of the bare optical fiber at an area where the sheath is removed is covered with a resin film having such a thinness to obtain an ultraviolet ray permeability not hindering formation of the grating, and the refractive index of an optical waveguide cyclically changes in the beam axis of the bare optical fiber by irradiating ultrasonic rays from the surface of the resin film.
Also, a fiber grating, according to the invention, is featured in that after a grating is formed on a bare optical fiber by irradiation of the ultraviolet rays, the outer circumferential surface of a resin film having an ultraviolet ray permeability is shielded with a reinforcing material.
According to the invention, since ultraviolet rays are irradiated from the surface side of a resin film after the surface side of a bare optical fiber is covered with the resin film having a thinness to obtain an ultraviolet ray permeability not hindering formation of the gratings, it is possible to prevent the bare optical fiber from being brought into contact directly with other components, and to prevent dust and/or foreign substances from being adhered to the surface of the bare optical fiber. As a result, it is possible to avoid damaging the surface of the bare optical fiber by the contact and/or dust or foreign substances. Still further, it is possible to avoid moisture adhering to the surface of the bare optical fiber and stress corrosion arising when irradiating ultraviolet rays.
Therefore, according to the invention, such conventional problems can be effectively prevented, in which the strength of the portions is lowered where the surface of a bare optical fiber is damaged and stress corrosion arose, and the long-term reliability of optical fiber type optical components in which fiber gratings are formed is lowered. Therefore, it is possible to produce optical components having strong fiber gratings and having excellent long-term reliability at a higher yield.
In particular, according to the invention in which a resin film is formed of an organic material having heat resistance by which the resin film is not melted by ultraviolet ray irradiation, since it is possible to prevent the resin film from deteriorating due to heat added to the resin film when irradiating ultraviolet rays and heat added to the resin film after formation of fiber gratings, optical components having greater strength fiber gratings with excellent long-term reliability can be produced.